Various processes of, and apparatuses for ink jet printing are now fairly well developed. For example, it is known that electrically conductive ink under pressure can be discharged through an orifice, broken-up into a stream of individual drops, and the charge or charge level on each of these discrete drops can be individually controlled. As the drops continue along a path toward the recording medium, such as a paper web, a pair of oppositely charged deflection plates positioned to either side of the ink drop path effect a deflection of each drop in accordance with its charge. If it is desired to leave a blank space on a portion of the medium, the charge level of that drop is controlled so its path terminates at a dump or an accumulator rather than at the record medium. This accumulator, dump or catcher includes a discharge channel for returing the ink to the reservoir or supply of ink under pressure. There are pressurized ink jet writing systems which do not break-up the stream into discrete droplets, but instead employ the stream or segments thereof. The present invention can be used to advantage in those systems too, if they include a dump displaced from the orifice for catching the ink which is directed so as not to mark the record medium.
Ink jet systems which are presently used release ink drops on start-up before the system is fully pressurized. Similarly, these systems have a transient decrease in ink pressure upon shutdown. The drops, expelled while the pressure is less than at the normal operating level, are not under proper control. Accordingly, those drops may impinge upon the components of the system, in one exemplary embodiment the charging unit, deflection unit, and the like. Though various systems for capping and uncapping the nozzle or ink ejection head are known in the art, the main objective of these systems is to prevent ink drying at the orifice from which the drops issue and for cleaning the orifice. Such systems concentrate on effecting a tight, even a hermetic, seal of the ink discharge orifice before start-up and after shut-down. Others effect a very rapid movement of a bayonet or shut-off plate into some portion of the ink path when the system is energized and de-energized. The prior art does not teach how to control the ink stream or drops in the gap between the nozzles and the record surface to prevent scatter or dispersion thereof on start-up and shut-down of the system.
It is, therefore, a primary object of the present invention to provide such an ink jet printing system in which unwanted dispersion is prevented both during start-up of the system and when the system is shut down.